docs: def _merge_data(dat1, dat2):
      """
      Merge two data dictionaries containing catalogue data
  
      :parameter dictionary dat1:
          Catalogue data dictionary
  
      :parameter dictionary dat2:
          Catalogue data dictionary
  
      :returns:
          A catalogue data dictionary containing the information originally
          included in dat1 and dat2
      """
  
      cnt = 0
      for key in dat1:
          flg1 = len(dat1[key]) > 0
          flg2 = len(dat2[key]) > 0
          if flg1 != flg2:
              cnt += 1
  
      if cnt:
          raise Warning('Cannot merge catalogues with different' +
                        ' attributes')
          return None
      else:
          for key in dat1:
              if isinstance(dat1[key], np.ndarray):
                  dat1[key] = np.concatenate((dat1[key], dat2[key]), axis=0)
              elif isinstance(dat1[key], list):
                  dat1[key] += dat2[key]
              else:
                  raise ValueError('Unknown type')
          return dat1